The present invention relates to a driving device for driving a liquid crystal display element employed in a liquid crystal display of a matrix electrode structure using a multiplex (time division) driving method, and more particularly, to a driving device for driving a liquid crystal display element by dividing a 1-line selection period into more than one segment.
Recently, liquid crystal displays are used in diversified fields including AV (Audio Visual) and OA (Office Automation) systems. Low-end products employ passive type liquid crystal displays, whereas high-end products employ liquid crystal displays driven by the active matrix driving method using switching elements, such as 3-terminal elements represented by TFTs (Thin Film Transistors) and 2-terminal elements represented by MIM (Metal-Insulator-Metal) elements.
Here, a conventional liquid crystal display will be explained with reference to FIG. 7. A display panel 102 of a conventional liquid crystal display 101 includes data electrode lines X1-Xn and scanning electrode lines Y1-Ym that intersect with the data electrode lines X1-Xn. Serially connected picture elements 103 are provided individually to portions enclosed by the data electrode lines X1-Xn and scanning electrode lines Y1-Ym. The picture elements 103 may include switching elements composed of the 2-terminal or 3-terminal elements.
A control section 104 of the liquid crystal display 101 receives an external interface signal IN from an unillustrated external circuit. For example, as shown in FIG. 8, the external interface signal IN includes a data signal DATA which conveys the display state of each picture element 103 in sync with a reference clock CLK, and a data enable signal ENAB which indicates whether the data signal DATA should be displayed or not. The external interface signal IN also includes a horizontal direction synchronizing signal LP supplied for every data signal DATA for each of the scanning electrode lines Y1-Ym, and a vertical direction synchronizing signal FP supplied for each screen (frame).
It is generally difficult to specify how many times the reference clock CLK is inputted in one cycle of the horizontal direction; synchronizing signal LP. This is because, in case of a display control circuit designed using, as a memory IC for storing the data signal DATA, a memory generally known as a DRAM which requires a refresh pulse, the frequency of the reference clock CLK varies with the specification of an external circuit (not shown) which generates an external interface signal.
The control section 104 generates a control signal which indicates a driving voltage and a driving timing of each of the data electrode lines X1-Xn and scanning electrode lines Y1-Ym in accordance with the external interface signal IN, and sends the same to a scanning electrode driving circuit 105 and a data electrode driving circuit 106. The scanning electrode driving circuit 105 selects the scanning electrode lines Y1-Ym successively in accordance with the control signal and applies a predetermined voltage to each. On the other hand, the data electrode driving circuit 106 applies a predetermined voltage to each of the data electrode lines X1-Xn in response to display data of the picture elements 103.
Here, a brief explanation of a voltage applied to one particular picture element 103 connected to a data electrode line Xi and a scanning electrode line Yj will be given with reference to FIGS. 9(a) through 9(e).
As shown in FIG. 9 (a), the horizontal direction synchronizing signal LP is applied to all the picture elements 103 for each of the scanning electrode lines Y1-Ym. Of the entire horizontal direction synchronizing signal LP, a period corresponding to the scanning electrode line Yj is a selection period for the subject picture element 103. An A/C signal M shown in FIG. 9(b) is generated based on the horizontal direction synchronizing signal LP of FIG. 9(a). The A/C signal M is a signal inverting periodically, for example, for every scanning electrode line.
During a non-selection period, that is, while the subject picture element 103 is not selected, the scanning electrode driving circuit 105 applies a voltage V1 or V4 to the scanning electrode line Yj as shown in FIG. 9(c) in accordance with the A/C signal M of FIG. 9(b). On the other hand, the data electrode driving circuit 106 selects a voltage to be applied to the data electrode line Xi depending on whether the subject picture element 103 connected to the currently selected scanning electrode line Yj and data electrode line Xi stays ON or OFF.
For example, as shown in FIG. 9(d), while the A/C signal M is in the high level, a voltage V0 indicated by a solid line is selected if the subject picture element 103 stays ON, and a voltage V2 indicated by a dotted line is selected if the subject picture element 103 stays OFF. On the other hand, while the A/C signal M is in the low level, a voltage V5 indicated by the solid line is selected if the subject picture element 103 stays ON, and a voltage V3 indicted by a dotted line is selected if the subject picture element 103 stays OFF. Consequently, as shown in FIG. 9(e), the voltage applied to the subject picture element 103 connected to the scanning electrode line Yj and data electrode line Xi varies within a range from a grounding level GND to a voltage Vb.
On the other hand, during the selection period, either a voltage V5 or V1 is applied to the scanning electrode line Yj in, response to the A/C signal M as shown in FIG. 9(c). Thus, in case that the data signal DATA conveys an ON command, as indicated by a solid line in FIG. 9(e), a voltage V0 is applied to the subject picture element 103 while the A/C signal M is in the low level, and a voltage xe2x88x92V0 is applied to the subject picture element 103 while the A/C signal M is in the high level, whereupon the subject picture element 103 comes ON. Likewise, in case that the data signal DATA conveys an OFF command, as indicated by dotted lines in FIG. 9(e), a voltage V2 is applied to the subject picture element 103 while the A/C signal M is in the low level, and a voltage xe2x88x92V2 is applied to the subject picture element 103 while the A/C signal M is in the high level, whereupon the subject picture element 103 goes OFF. Consequently, both the driving circuits 105 and 106 can drive the picture elements 103 individually by the voltage averaging method.
Incidentally, the characteristics of the liquid crystal display vary with a change of environmental conditions, such as temperatures, irregular characteristics of the display panel per se as an electronic component, etc.
For instance, it is known that a display quality, particularly, the contrast, of a liquid crystal display using the 2-terminal elements depends largely on ambient temperature. FIG. 10 shows V-CR (voltage-vs.-contrast) characteristics of the liquid crystal display using the 2-terminal elements. In the drawing, a curve (a) represents the characteristics at normal temperature, a curve (b) represents those at high temperatures, and a curve (c) represents those at low temperatures. FIG. 10 reveals that, when the temperature is low, a maximum contrast value and a liquid crystal applying voltage (hereinafter, referred to as maximum contrast voltage) necessary to obtain the maximum contrast value are larger compared with those at normal temperature. FIG. 10 also reveals that, when the temperature is high, both the maximum contrast value and maximum contrast voltage are smaller compared with those at normal temperature.
Therefore, when the temperature is high, the maximum contrast value becomes too small, whereas when the temperature is low, the maximum contrast voltage becomes so large that it exceeds a specification value of a withstand voltage for a liquid crystal, driver IC. Hence, there arises a problem that the voltage alone can not adjust the driving voltage conditions to optimal ones to obtain satisfactory contrast in a broad range of temperatures.
A technique for effecting temperature compensation by controlling pulse duty while maintaining the maximum contrast voltage is disclosed in, for example, Japanese Laid-Open Patent Application No. 116792/1984 (Tokukaisho No. 59-116792).
According to the above technique, when a voltage corresponding to an ON signal for a selection period is applied, the voltage is not applied in a straight forward manner. Instead, a period for applying the voltage corresponding to the ON signal and a period for applying a voltage corresponding to an OFF signal are provided in a mixed manner to control the pulse duty. The pulse duty throughout the selection periods, during which the voltage corresponding to the ON signal is applied, is controlled in the following manner. That is, a temperature compensating circuit generates a pulse width signal that varies with environmental temperatures, based on which the period for applying the voltage corresponding to the ON signal to each picture element is extended when the temperature is low and shortened when the temperature is high.
The above arrangement of the disclosed technique makes it possible to compensate the temperature dependency of the characteristics of the liquid crystal display element without adjusting the maximum contrast voltage of the liquid crystal display.
In a conventional driving device for driving the liquid crystal display element without changing the pulse duty, the maximum contrast value to achieve an optimal display quality is attained by merely adjusting the liquid crystal driving voltage. To obtain a satisfactory contrast value in a broader range of temperatures, the driving device only has to be modified in such a manner that a higher liquid. crystal driving voltage can be applied. However, there occurs a problem that, if the liquid crystal driving voltage is raised too high, the liquid crystal driving voltage value exceeds a specification value of a withstand voltage for a liquid crystal driving driver IC. This problem could be solved by composing the liquid crystal driving driver IC with component parts which can withstand high voltages. However, in this case, there occurs another problem that a technical breakthrough must be made for producing such a liquid crystal driving driver IC which can withstand high voltages. Moreover, since the component parts are not readily available; even if the liquid crystal driving driver IC which can withstand high voltages is successfully produced, the cost thereof is too high.
The driving method, disclosed in aforementioned Japanese Laid-Open Patent Application No. 116792/1984 (Tokukaisho No. 59-116792), for effecting the temperature compensation by changing the pulse duty can only realize the adjustment by the liquid crystal driving voltage in substantially the same range of temperatures as the range attained by the conventional driving device for driving the liquid crystal display element without changing the pulse duty. The reason is as follows.
That is, to leave a margin for effecting a pulse width control, a pulse width at normal temperature for applying the voltage corresponding to the ON signal must be set shorter than the selection period. Thus, compared with a case where the pulse width control is not effected, the picture elements must be charged in a shorter time, and for this reason, a higher liquid crystal driving voltage needs to be applied. Accordingly, when the method for effecting the pulse width control is adopted, there arises a problem that the liquid crystal driving driver IC must withstand higher voltages compared with its counterpart employed in the conventional driving. device for driving the liquid crystal display element without changing the pulse duty.
It is therefore an object of the present invention to provide a driving device for driving a liquid crystal display element which can effect a pulse width control that offers an advantage of maintaining a small liquid crystal driving voltage, while compensating the characteristics of the liquid crystal display element in a broader range of temperatures by adjusting the magnitude of the liquid crystal driving voltage.
To fulfill the above and other objects, a driving device for a liquid crystal display element of the present invention, which divides a selection period for setting a display state of a picture element having a 2-terminal element into a plurality of segments and applies different voltages to the picture element in each of the plurality of segments, is characterized by being furnished with:
a dividing section for receiving a logical signal which varies with use environment of the liquid crystal display element, then selecting one dividing ratio pattern from at least two different predetermined dividing ratio patterns in accordance with the logical signal, and dividing the selection period into the plurality of segments based on a selection result, wherein,
the picture element is charged by being applied with a predetermined voltage during at least one segment out of the plurality of segments made by the dividing section whether the picture element stays ON or OFF, and
charges supplied to the picture element are discharged during at least another segment out of the plurality of segments other than the above one segment in accordance with an ON/OFF state of the picture element.
According to the above arrangement, one dividing ratio is selected from the plurality of predetermined dividing ratio patterns for the selection period in accordance with the logical signal which varies with environment conditions (environmental temperature, for example). Here, the logical signal is defined as a signal obtained by analog-to-digital conversion of an output from a temperature sensor or the like. The dividing ratio patterns are set in such a manner to extend an initial charging period and shorten a following discharging period when the picture element using the 2-terminal element of the liquid crystal display has hard-to-charge and easy-to-discharge characteristics, and to shorten the initial charging period and extend the following discharging period when the picture element has easy-to-charge and hard-to-discharge characteristics.
If the 2-terminal element per se serving as an electronic component at normal temperature has the above-mentioned characteristics, it can be used as an adjusting section. In this case, in response to the characteristics of the 2-terminal element with respect to the environmental temperature, that is, if it is hard to charge and easy to discharge when the temperature is high and easy to charge and hard to discharge when the temperature is low, a predetermined initial voltage is applied to the picture element in such a manner to extend a period for charging the picture element, and a period for discharging the picture element is shortened, whereas when the temperature is high, the predetermined initial voltage is applied to the picture element in such a manner to shorten the period for charging the picture element, and the period for discharging the picture element is extended.
According to the above driving, the voltage-vs.-contrast characteristics in each temperature range can be approximated to those at normal temperature by applying a suitable dividing ratio pattern for the selection period in each temperature range. In other words, satisfactory display contrast can be obtained in a broad range of temperatures by switching the dividing ratio pattern for the selection period in each temperature range.
Also, in the above driving, unlike the case of effecting the pulse width control disclosed in aforementioned Japanese Laid-Open Patent Application No. 116792/1984 (Tokukaisho No. 59-116792), a voltage value of the driving voltage applied to each picture element during the selection period can be set to a value as small as the one in the case where the pulse width control is not effected. Thus, since the driving device for the liquid crystal display element of the present invention has a margin for a higher driving voltage, the driving voltage can be adjusted at the same time.
It is preferable to arrange the driving device for the liquid crystal display element of the present in such a manner that the dividing section includes a timing signal generating section for generating a timing signal which divides selection periods following the above selection period into the plurality of segments in accordance with a signal indicating the above selection period and a reference clock signal in sync with the above selection period and having a cycle shorter than the above selection period,
wherein the timing signal generating section is composed of:
a counter for counting a reference clock in accordance with the signal indicating the above selection period; and
a plurality of gates for receiving a pre-set clock value for the dividing ratio selected and a count value of the counter, and for generating the timing signal when the count value reaches the pre-set clock value,
at least one of the plurality of gates being selected in accordance with the logical signal when the timing signal is generated.
According to the above arrangement, in case that the number of the reference clock for one selection period is limited to a specific value, the counter counts the number of the reference clock for one selection period in accordance with the signal indicating the above selection period (for example, a horizontal direction synchronizing signal), and outputs the count value to the gates. The gates also receive a predetermined clock value for the selected dividing ratio so as to generate the timing signals when the input count value reaches the predetermined clock value. The gates further receive the logical signal and at least one of the gates is selected by the same.
Accordingly, the driving device for the liquid crystal display element of the present invention can supply a stable timing signal in a circuit of a minimum size (least number of the gates) by means of a logical circuit alone. For example, the driving device for the liquid crystal display element of the present invention is applicable to a case where the specification for an external interface signal from an external circuit is limited to one kind, such as a television standard represented by NTSC and in the field of portable information terminals. In other words, the driving device for the liquid crystal display element of the present invention is, applicable in the field of OA systems where the liquid crystal display is designed as an integral component part of a system unit, thereby making it possible to downsize the liquid crystal display while saving the costs.
It is preferable to arrange the driving device for the liquid crystal display element of the present invention in such a manner that the dividing section includes:
a parameter computing section for computing a parameter used for dividing selection periods following the above selection period in accordance with a signal indicating the above selection period and a reference clock signal in sync with the above selection period and having a cycle shorter than the above selection period;
a storage section for storing the parameter; and
a timing generating section for generating a timing signal out of the signal indicating the above selection period and a reference clock signal, the timing signal dividing selection periods following the above selection period into the plurality of segments in accordance with the parameter,
the parameter computing section including a plurality of sections respectively corresponding to the dividing ratio patterns, one of the plurality of the sections being selected in accordance with the logical signal when the timing signal is generated.
According to the above arrangement, the parameter computing section computes the parameter for the liquid crystal display element in accordance with the reference clock signal before the timing generating section generates the timing signal. The timing generating section generates the timing signal based on the above parameter.
Accordingly, the driving device for the liquid crystal display element of the present invention can divide each selection period with a desired dividing ratio regardless of the number of the reference clocks for one selection period. Further, the driving device for the liquid crystal display element of the present invention can operate on the specification for an external interface signal having a different number of the reference clocks for one selection period and sending a signal that determines the selection period at different timing. In other words, the driving device for the liquid crystal display element of the present invention can operate with an external circuit which generates an external interface signal to be sent to the liquid crystal display in accordance with a different specification.
In the field of general OA systems, such as personal computers, where most of the external circuits generally are display control circuits developed by a third party, the specifications of the external interface signals outputted from each display control circuit are slightly different one from another. However, the driving device for the liquid crystal display element of the present invention can operate with the display control circuit of any kind, and therefore can be shared among the users. In short, the driving device for the liquid crystal display element of the present invention has versatility and is expected to offer advantages of mass production.
In addition, the driving device for the liquid crystal element of the present invention uses only the signals supplied to the liquid crystal display from the external circuit without fail as the external interface signal, whereas the signal that determines the dividing timing of the selection period is generated inside the driving device for the liquid crystal element of the present invention. Thus, it is not necessary to separately provide the signal for dividing the selection period to the driving device form the liquid crystal display element of the present invention from the external circuit.
Consequently, the driving device for the liquid crystal display element of the present invention can maintain the interface. with the external circuit in the same manner as the conventional liquid crystal display which does not divide the selection period. Thus, the driving device for the liquid crystal display element of the present invention can use an external display control circuit designed for the conventional driving device for the liquid crystal display element which does not divide the selection period.
Moreover, the driving device for the liquid crystal display element of the present invention can omit additional circuit components for generating the signal which determines the dividing timing of the selection period, such as quartz oscillator and a PLL (Phase Locked Loop) circuit, thereby realizing a simple, downsized, less expensive, and less power-consuming driving device for the liquid crystal display element.
It is preferable to arrange the driving device for the liquid crystal display element of the present invention in such a manner that the logical signal is inputted through an external switch provided to a main body of a system employing the liquid crystal display element, the external switch enabling a user to select the dividing ratio for the selection period directly.
According to the above arrangement, the logical signal is inputted through the external switch provided to the system main body, so that the user can select any dividing ratio for the selection period by himself. Also, since the user adjusts the dividing ratio with the driving device for the liquid crystal display element of the present invention, an expensive and large-space occupying component, such as a temperature compensating circuit, can be omitted, thereby making it possible to provide an inexpensive and simple temperature compensating section.
It is preferable to arrange the driving device for the liquid crystal display element of the present invention in such a manner that the logical signal is inputted through a logical circuit which selects a dividing ratio suitable for environmental conditions predicted from regional information, calendar information and use environmental information, under which a main body of a system employing the liquid crystal display element is used.
According to the above arrangement, the system main body can learn whether the temperature conditions in the environment in which the system main body is used are those in the summer time in a range between normal temperature (about 20xc2x0 C.) and high temperature (40xc2x0 C. or higher), or those in the winter time in a range between normal temperature and low temperature (about 0xc2x0 C.). Thus, the driving device for the liquid crystal display element of the present invention can set the operation environmental conditions by selecting a pulse width dividing ratio suitable to the environmental conditions predicted from the aforementioned information as the pulse width dividing ratio to be applied to each picture element, thereby making it possible to compensate the operation of the liquid crystal display element. In addition, since the system main body adjusts the dividing ratio based on the pre-stored information, an expensive and large-space occupying component, such as a temperature compensating circuit, can be omitted. Consequently, a simple and inexpensive temperature compensating section can be provided.
It is preferable to arrange the driving device for the liquid crystal display element of the present invention in such a manner that the logical signal changes the dividing, ratio for the selection period during the operation.
According to the above arrangement, the driving device for the liquid crystal display element of the present invention can change the dividing ratio while the power source stays ON, without initializing the liquid crystal display by turning OFF the power source. Thus, the pulse width dividing ratios can be compared with each other more readily, and the driving conditions can be adjusted to the optimal ones more easily.
The driving device for the liquid crystal display element of the present invention can be arranged in such a manner that the timing generating section changes the dividing ratio for the selection period per frame during the operation.
According to the above arrangement, the driving device for the liquid crystal display element of the present invention can change the dividing ratio for the selection period immediately after the number of the reference clocks for the selection period is changed.
Consequently, the driving device for the liquid crystal display element of the present invention can constantly drive the liquid crystal display element under optimal driving conditions.
The driving device for the liquid crystal display element of the present invention can be arranged in such a manner that the timing generating section changes the dividing ratio for the selection period once in every certain number of frames.
According to the above arrangement, the driving device for the liquid crystal display element of the present invention saves the power consumption of the entire system while driving the display liquid crystal element under the optimal driving conditions by changing the dividing ratio for the selection period once in every certain number of frames.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.